The present invention relates to a method for determining a failure in an EGR (exhaust gas recirculation) apparatus.
The major ingredients of the exhaust gas discharged from a gasoline engine are carbon monoxide (CO), hydrocarbon (HC), and nitrogen oxides (NOx). Nitrogen oxides are produced by the chemical reaction between nitrogen and oxygen contained in an air-fuel mixture under a high-temperature condition which takes place when the air-fuel mixture supplied to an engine burns. The majority of nitrogen oxides contained in the exhaust gas is nitric monoxide (NO). Even with the same air-fuel ratio of the air-fuel mixture, if the quantity of inactive ingredients contained in the air-fuel mixture increases, the combustion temperature of the air-fuel mixture lowers with consequent reduction in the nitric monoxide produced when the air-fuel mixture burns.
Based on the fact described above, an EGR apparatus designed to cause part of exhaust gas to be returned to an induction system of an engine to thereby add the exhaust gas to an air-fuel mixture as an inactive ingredient is used for exhaust gas purification.
Further, as a method for diagnosing a failure of an EGR apparatus, "METHOD FOR DIAGNOSING A FAILURE OF AN EXHAUST GAS CIRCULATION CONTROLLER" which performs a failure diagnosis when an engine is running in a decelerated operation zone where fuel supply is cut is disclosed in Japanese provisional patent publication No. H2-9937. According to this diagnosis method, to perform the failure diagnosis, when an engine is in a stable condition following the completion of warm-up, a negative-pressure selector valve provided in a negative-pressure passage is temporarily changed over from an open state to a closed state to thereby introduce an intake pressure into the negative-pressure passage. The intake pressure thus introduced acts on a negative-pressure chamber of an EGR valve, which is disposed in an EGR passage, via an EGR modulator, thereby causing the EGR valve to open to circulate the exhaust gas from an exhaust passage of the engine to an intake passage of the engine via the EGR passage. Then, a difference between the intake pressure developed immediately before the exhaust gas recirculation (EGR) and that developed during the EGR is detected. If the difference is below a preset value, then it is determined that a failure of the EGR apparatus has occurred.
More specifically, according to the aforesaid failure diagnosis method, when a throttle valve is fully closed (FIG. 1A), an EGR check mode is turned ON and check (failure diagnosis) of the EGR apparatus is started (FIG. 1B). Exhaust gas recirculation (EGR) is forcibly carried out between the moment a prescribed time elapses from the time at which the EGR check mode is turned ON and the moment the checking of the EGR apparatus is completed (FIG. 1D). It is determined that the EGR apparatus is working properly, if a variation .DELTA.P of the intake pressure (intake manifold pressure) between the moment immediately before the EGR is performed and the moment the EGR is being carried out is large, as indicated by a solid line in FIG. 1C. On the other hand, if the variation a .DELTA.P in the intake manifold pressure is small as indicated by the two-dot chain line in FIG. 1C, then it is determined that a failure has occurred in the EGR apparatus.
According to the aforesaid failure diagnosis method, however, in some cases, accurate determination of the occurrence/absence of a failure in the EGR apparatus cannot be performed.
Causes of such erroneous determination include fluctuations in the intake manifold pressure which result from a change in the engine speed during the EGR check. To be more specific, during the EGR check, if the engine operation state shifts from point A to point B on the engine speed vs. intake manifold pressure characteristic curve obtained when the throttle is fully closed (FIG. 2), causing the engine speed Ne to drop from a value Ne.sub.A to a value Ne.sub.B, then the intake manifold pressure increases from point A to point B on the solid line shown in FIG. 1E. If the increase in the intake manifold pressure due to the drop in the engine speed during the EGR check is large, and hence the variation of the intake manifold pressure detected for the EGR check is large, then the EGR apparatus may be erroneously determined to be working properly even when the EGR apparatus has developed a failure. If the engine operation state shifts from point B to point A on the aforesaid characteristic curve during the EGR check, so that the engine speed Ne increases from the value Ne.sub.B to the value Ne.sub.A, then the intake manifold pressure changes from point B to point A on the broken line, as shown by the broken line in FIG. 1E. Accordingly, if the variation of the intake manifold pressure, which is detected while the intake manifold pressure is changing under an influence exerted by the increase in the engine speed, is small, the EGR apparatus may be erroneously determined to be faulty even when the EGR apparatus is working properly.
Furthermore, in the aforesaid conventional failure diagnosis method, an engine operation mode switching between a fuel-cut mode and a non-fuel-cut mode may lead to erroneous determination. This is because the combustion of an air-fuel mixture is started or stopped when the fuel-cut mode is released or entered during the EGR check, to cause the intake manifold pressure to be changed.
Specifically, if the EGR check is initiated when the engine is in an operation state which corresponds to point A on the engine speed vs. intake manifold pressure characteristic curve, shown by the solid line In FIG. 3, for the fuel-cut mode, and if the EGR check is completed when the engine is in an operation state which corresponds to point A' on the characteristic curve, shown by the broken line in FIG. 3, for the non-fuel-cut mode, then the intake manifold pressure increases from point A to point A' on the solid line as indicated by the solid line in FIG. 4 as time elapses. If the increase in the intake manifold pressure due to the mode changeover is large and the resulting variation of the intake manifold pressure detected for the EGR check is large, then it may be erroneously determined that the EGR apparatus is working properly while the EGR apparatus actually has incurred a failure. On the other hand, if the EGR check is started with the engine operation state corresponding to point A' in FIG. 3 and completed with the engine operation state corresponding to point A in FIG. 3, then the intake manifold pressure changes from point A' to point A on the broken line, as shown by the broken line in FIG. 4. As a result, the variation of the intake manifold pressure which is detected for the EGR check decreases, and it may be erroneously determined that the EGR apparatus has failed even when it is working properly.
Further, when the conventional failure diagnosis method described above is applied to an engine provided with an idling speed control (ISC) function of automatically adjusting the idling speed in accordance with engine load, the idling speed adjustment performed by the ISC function may lead to erroneous determination. This is because the intake manifold pressure changes when the opening degree of the ISC valve, which is provided in a bypass passage which bypasses a throttle valve, is changed to adjust the idling speed.
Meanwhile, the idling speed adjustment performed by the ISC function is intended to prevent engine stall when engine load increases due to, for example, the actuation of an air conditioner or a power steering pump. When the engine load increases (during idle-up), the opening degree of the ISC valve is increased to supply secondary air to the engine via the bypass passage, thereby running the engine at an idling speed which is higher than an ordinary low idling speed.
The following explains the reason why the idling speed adjustment by the ISC function causes the erroneous determination in the failure diagnosis of the EGR apparatus.
If the EGR check is started when the engine is in the operation state corresponding to point A on the engine speed vs. intake manifold pressure characteristic curve (shown in FIG. 5) obtained when the ISC valve is set at a reference opening degree, and if the EGR check is completed when the engine is in the operation state corresponding to point A' on a similar characteristic curve obtained when the idle-up is carried out, then the intake manifold pressure increases from a value (approximately -520 mm Hg) corresponding to point A in FIG. 5 to a value (approximately -510 mm Hg) corresponding to point A' in FIG. 5 as the opening degree of the ISC valve increases for the idle-up. If the increase in the intake manifold pressure resulting from the change in the ISC valve opening degree is significant with a consequent great variation of the intake manifold pressure, which is detected for the EGR check, then it may be erroneously determined that the EGR apparatus is working properly even when the EGR apparatus has actually incurred a failure. On the other hand, if the EGR check is started with the engine being in an operation state corresponding to point A' of FIG. 5 and completed with the engine being in an operation state corresponding to point A of FIG. 5, then the intake manifold pressure decreases from a value corresponding to point A' to a value corresponding to point A. As a result, the variation of the intake manifold pressure, which is detected for the EGR check, is small, and hence it may be erroneously determined that the EGR apparatus has failed even when it is actually working properly.
As described above, according to the conventional failure diagnosis method, changes in the engine operation states (e.g., engine speed, fuel supply to the engine, and secondary air supply to the engine) other than the intake state in the intake passage of the engine prevents accurate determination of the occurrence/absence of a failure in the EGR apparatus.